Those colours aren't too bad, though they are relatively light for them to show up well on a lot of skins. I find Light blue a rather nice choice of colour for Supporters. What with it being of the lighter shade accompanied with the italicised font, I doubt there'd be much trouble with telling it apart from the current moderator Steelblue. But of course that can still be left open to debate.
All I'm aware of is that having an only italicised username is the most convenient way, aside from the colour, to represent a user whom has donated to the forums. The colour wouldn't be able to show too much differentiation from a regular username unless its a vivid hue, to which you will always find people who don't like colours that are too light, too dark, too hard on the eyes, etc. Plus if you were to bring up the idea of having bold alone without any colour, then people would easily mistake you for someone with a higher status. Bear in mind that this is only just my input, but to put it simply; I find italic usernames to hold more definition above anything else.